The proposed studies have two purposes: 1) The binding of the Alpha-neurotoxin, Alpha-bungarotoxin, (AlphaBTX), to specific receptors within the suprachiasmatic nucleus (SCN) of the hypothalamus will be investigated by electron microscopic autoradiography in the intact, chronically ovariectomized, and ovariectomized and estrogen-treated rat. This study is proposed because we have observed that in intact females, males, and in castrated male rats, the SCN binds (125I)AlphaBTX, but in chronically oophorectomized rats, the labeling of the neurotoxin is markedly reduced. Since AlphaBTX presumably binds to a significant degree to the postsynaptic membrane, the ultrastructure of these synapses in the intact and chronically oophorectomized and ovariectomized-estrogen treated animals will be compared on a morphological basis. 2) The second portion of the work is designed to localize choline acetyltransferase (CAT), the synthetic enzyme for acetylcholine, within the SCN by using a peroxidase-antiperoxidase (PAP) technique and a monoclonal antibody to the enzyme. Localization of CAT, a presynaptic marker1, will be performed using the following animal models: a) To test whether or not retinohypothalamic afferents to the SCN may be cholinergic in nature, the location of CAT in normally cycling, normally cycling and orbitally enucleated rats, and rats exposed to constant light will be compared by the PAP method and light microscopy. b) To test the effect of gonadal steroids on CAT in the SCN, normally cycling, chronically oophorectomized, and oophorectomized animals which receive estrogen replacement will be examined by light microscopy. In sum, these studies will further our understanding of hypothalamic nicotinic cholinergic receptors and their relationship to gonadal steroids.